Optoelectronic circuits have a wide range of applications, from signal processing to computing. Example applications include coherent and incoherent signal processing, optical filtering, RF filtering, switching, and modulation. The potential bandwidth, speed and other attributes of optical circuits will continue to increase their applications.
An exemplary application is to microwave circuits, e.g., the circuits used in wireless handsets to send and receive wireless network communications. The basic problem is one of simple filtering, there is a need to tune the circuit to filter a specific range around a center frequency, or frequencies, corresponding to one or more bands of operation. Tolerances in typical manufacturing processes for microwave tuning circuits lead to difficulties. The exact size, coupling efficiency, chemical composition, etc. of waveguides in microwave circuits can lead to differences in the center frequency of circuits manufactured in accordance with a particular design. Re-tuning of a manufactured optical component may require physical changes and also may not be possible within the permissible design range for a given center frequency. After the manufacture of a typical optical tuning circuit, it is difficult to modify the response frequency of the circuit.
Optical delay lines have been used to address such problems, and similar problems in other applications. One type of optical delay line provides a number of different path lengths to achieve different amounts of delay. Typically, micromechanical switches are used to switch out different path lengths and achieve a given delay. Heating is another method to change the frequency of a given waveguide. Heating of a waveguide can change the path length of the waveguide. However, heating has a slow response time, and also may not be practical as a solution in all applications.
Low loss waveguide loop or ring structures are have been used in coherent and incoherent optical signal processing, e.g., optical or RF filtering, switching, and modulation. Passive or active loops have been fabricated in a number of waveguide materials, e.g., glass waveguides, polymethyl methacrylate (PMMA), GeO2-doped silica, and conventional optical fibers. The resonance frequency in these ring structures generally cannot be tuned conveniently. Some can be tuned thermally, however, the tuning speed and accuracy are not sufficient for high speed applications.
Another interesting structure in optoelectronics is the 1 to N splitter. Such splitters are used in many signal processing applications. An example application is for label recognition of photonic packet switching networks. Takahashi, et al., “40-Gbit/s Label Recognition and 1×4 Self-Routing Using Self-Serial-to-Parallel Conversion”, IEEE Photon. Technol. Lett., Vol. 16, pp 692-94 (February 2004). Another application for such a splitter is to conduct data rate conversion for large-capacity storage networks. Suzuki et al., “Ultrafast Photonic Interfaces for Storage Networking Using Serial-to-Parallel and Parallel-to-Serial Conversion”, Proc. of SPIE, Vol. 5069, pp 35-44 (2003). An additional splitter application is an all-optical register. Lugagne, et al., “Operation of 4×1 Optical Register as a Fast Access Optical Buffer Memory,” Electron. Lett., Vol. 33, pp 1161-62 (June 1997). Splitters may also be used in optical RF beam forming. Esman et al., “Fiber-Optic Prism True Time-Delay Antenna Feed”, IEEE Photon. Technol. Lett., Vol. 5, pp 1347-1349, 1993. Splitters have been created using surface-emitting planar light wave circuits, as in Takashi and Suzuki (supra), using an acousto-optic modulator, as in Lugagne (supra), or using a fiber-based technique, as in Esman (supra). None of these splitting techniques admits readily of re-programming or fine-tuning to meet the dynamic variation of networks, however.